Police have upgraded the charges against a young man accused of a deadly one-punch assault at a Western Sydney party.

Hugh Garth, 21, allegedly hit Raynor Manalad, also 21, at a birthday party at Rooty Hill last Saturday morning.

He has now been charged with assault causing death while intoxicated and causing grievous bodily harm, making him the first person to be dealt with under new one-punch mandatory sentencing laws in New South Wales.

A second charge of grievous bodily harm was added as police allege Garth struck another person at the party in the minutes before he allegedly hit Mr Manalad.

The one-punch mandatory sentencing laws, which came into effect in January, place an eight-year minimum prison sentence on anyone found guilty of hitting someone with a single punch while drunk or on drugs.

Police allege Mr Malanad tried to intervene in an argument between Garth and his girlfriend, and was hit in the head by Garth.

Mr Manalad reportedly fell after being hit and struck his head on the pavement.

He was taken to Westmead Hospital but never regained consciousness and died on Sunday evening.

Today Garth appeared via videolink at Blacktown Local Court. He did not apply for bail and it was formally refused.

Speaking outside court, family friend Jodie Webster said she can not believe Garth was responsible.

"He would never ever do this, he'd never hurt anybody on purpose," she said.

"I've looked after this boy for a long time and he's a beautiful person you know he's got a good soul and he'd never do this."